1. Field of the Invention
This invention relates to foamable preparations based on organosilicon compounds, to silicone-containing polyurethane foams, in particular molded foams, having low densities, and also to processes for production thereof.
2. Description of the Related Art
Polyurethane foams are generally prepared by reaction of a polyisocyanate with compounds containing two or more active hydrogen atoms. The compounds containing active hydrogen are typically polyols, primary and secondary polyamines, and water. Between these reactants there are two principal reactions that occur during the preparation of a polyurethane foam. These reactions must in principle run simultaneously and with a competitively balanced rate during the operation, in order to produce a polyurethane foam having desired physical properties. The reaction between the isocyanate and the polyol or polyamine, which is typically termed a gel reaction, leads to the formation of a polymer with a high molecular weight. The progress of this reaction increases the viscosity of the mixture and contributes generally to the formation of crosslinking with polyfunctional polyols. The second principal reaction takes place between the polyisocyanate and water. This reaction contributes to the growth of the urethane polymer and is important for the formation of carbon dioxide gas, which assists the foaming process. Consequently this reaction is often termed the blowing reaction. Both the gel reaction and the blowing reaction take place in foams which are blown partially or completely with carbon dioxide gas. If, for example, the evolution of carbon dioxide is too rapid by comparison with the gel reaction, the foam exhibits a proclivity to collapse. If, alternatively, the gel expansion reaction is too rapid as compared with the blowing reaction that produces carbon dioxide, foam rise is limited, and a high-density foam is produced. Similarly, poorly matched crosslinking reactions will impact adversely on foam stability.
The polyols used are generally polypropylene glycols, which in accordance with the prior art can be prepared in a very wide variety of topologies, and differ from one another in molecular weight, degree of branching, and OH number. In spite of the broad structural variation of these polyols and the associated tailoring of the polyurethane foams to virtually any application, the inherent flammability of the commercially available polyurethane foams is a serious drawback. In spite of great efforts, success has so far not been achieved in establishing absolutely inflammable flexible PU foams on the market, although in recent decades there has been no lack of intense research activities aimed at improving the flame retardancy properties of polymer foams.
One route to flame-retarded, flexible PU foams is taken in silicone-polyurethane flexible foams. In such foams, the highly combustible polyol component that is used in standard PU foams is replaced by incombustible, OH-terminated siloxanes. Through the use of silicone-polyurethane copolymers, i.e., of polysiloxanes, which also contain polyurethane units and/or urea units, it is possible to develop incombustible foam materials of this kind which have new combinations of properties that are tailored precisely to the particular application. Reference on this point may be made, for example, to EP 1485419 B1, which describes the preparation of silicone-polyurethane foams starting from alkylamino- or alkylhydroxy-terminated silicone oils and diisocyanates in what is called a “one-shot” process. Furthermore, DE 102006013416 A1 describes the preparation of silicone-PU foams from prepolymers which are prepared in a solvent-based operation on the basis of alkylamino- or alkylhydroxy-terminated silicone oils and diisocyanates.
A feature which unites the silicone-polyurethane foams that have been described to date is that they are prepared on the basis of siloxanes which are linear or have only very slight, but statistical, branching in the side chains. In view of this linear siloxane chain, the rise phase during foaming is not accompanied by an increase in molar mass, and so the increase in viscosity during the rise phase is relatively slow, meaning that the polymer matrix, even after the end of the blowing reaction, is generally slightly fluid, and, therefore, the fine cell structure may still collapse before curing of the foam is complete. Even if only a small fraction of the cell structure collapses in on itself, the result is a coarse and irregular cell distribution. In order to counteract cell collapse when using linear polyol components, the struts connecting the individual foam cells must not fall below a critical diameter during the rise phase. Hence it is ensured that the still fluid polymatrix is able to counteract the threat of collapse of the foam structure. If, however, the desired foam density selected is too low, then the cell struts become increasingly thin during the rise phase until, finally, they become too flexible to stabilize the cell structure. Accordingly, in general, linear siloxanes result only in silicone-PU foams having densities of distinctly above 100 kg/m3.
A further disadvantage with the silicone-PU foams described to date is that NCO-terminated silicone prepolymers have to be used if silicone-PU foams having low densities are to be obtained. The preparation of appropriate prepolymers requires an additional step of synthesis and, moreover, such prepolymers have but limited stability in storage at elevated temperatures in particular. It would accordingly be desirable to have a process whereby the classic one-shot method can be utilized in foam production. In such a process, the polyol and isocyanate parts would be prepared independently of each other and would only be made to react with each other in the foaming operation.
In addition, the known NCO-terminated silicone prepolymers cannot be used to produce molded foams having optimal properties, since the molded foams obtained therewith have very coarse and irregular cells directly under the skin, creating the haptic impression of inferior quality. It is accordingly desirable to be able to produce silicone-PU foams to the same quality as conventional molded polyurethane foams. For this they need to have a completely uninterrupted and homogeneous surface which transitions directly into the fine-cell structure in their interior.